Nitrocellulose coating composition for metals



Patented July 2, 1957 NITROCELLULOSE COATING COMPOSITION FOR METALS Gregory N. Bruxelles, Newport, Del., assignor to Hercules Powder Company, Wilmington, Del., a corporation ct Delaware No Drawing. Application June 7, 1954, Serial No. 435,066

2 Claims. (Cl. 148-6.16)

This invention relates to a coating composition for metals and more particularly to a coating composition which has improved adhesion to metals, the ability to inhibit the corrosion of metals and wherein nitrocellulose is the film-forming ingredient.

The chemical treatment of metal surfaces to render them resistant to corrosion and provide a suitable surface to which a protective coating will adhere has been the object of considerable investigation. Such chemical treatment is commonly accomplished by the use of wash primers which are organic coatings applied in very thin films, in the order of 0.3 mil, as a pretreatment conditioner for subsequent painting. In view of the thinness of the coating, it is quite important that the film produced is uniform and continuous. If a nonuniform and discontinuous fihn is produced due to contamination or lack of good film properties for any reason, the composition has f sultmg composition was cooled to room temperature.

little value or at best is inferior as a wash primer.

Three of the most difli'cult properties to obtain in organic coatings for metal are adequate adhesion, impact resistance, and prevention of under film corrosion of the metal. Numerous treatments and coatings have been developed in the past in an endeavor to obtain these desired properties. They include phosphoric acid treatment (e. g., Bonderizing), zinc chromate primers, red lead primers, and phenolic resin primers.

Wash primers, which represent both a metal treatment and an organic coating, are one of the most recent methods for improving metal coatings. The earliest wash primer, a two-package system, was called the WP-1 type. This system employed zinc chromate, polyvinyl butyral, and solvents in one package, and a phosphoric acid diluent in the other. This wash primer had poor can stability (about eight hours), was expensive, and its film was prone to blister under conditions of high humidity. An improved wash primer called the chromic-acid type was later developed. This primer consisted of an alcoholic solution of polyvinyl butyral reacted at elevated temperature with phosphoric and chromic acids. This later chromic acid wash primer has advantages over WP-l primers in that can stability was improved (over one year) as well as resistance to humidity. The major disadvantages to the chromic acid wash primers now commercially available are the high cost of raw materials, the hazardous chemicals used in their manufacture, and their susceptibility to contamination which usually results in poor performance.

With these disadvantages in view, it is, therefore, a primary object of this invention to provide an improved corrosion inhibiting coating or wash primer of the chromic-acid type for metals including steel, aluminum, and the like wherein relatively low cost raw materials are utilized which are readily available and which may be reacted with satisfactory results to produce a wash primer having good properties in respect to impact resistance, scratch adhesion of subsequent coatings and resistance to contamination, which properties additionally contribute to improving the corrosion resistance of the wash primer.

' phosphoric acid to 90 parts of acetone.

Other objects of the invention will appear hereinafter, the novel features and combination being set forth in the appended claims.

According to this invention, there is provided a coating composition characterized by improved adhesion to metals and the ability to inhibit the corrosion of metals, com prising the reaction product of a solution of chromium trioxide, phosphoric acid and nitrocellulose dissolved in organic solvent.

The following example is illustrative of the invention wherein all parts given are parts by weight.

Example A first solution was prepared by adding 33.4 parts of chromium trioxide to 66.6 parts of water. A second solution was prepared by adding 10 parts of ortho- Sti'll a third solution was prepared by adding 14.7 parts of SS nitrocellulose (5-6 see. dry) to a solvent mixture consisting of 12.4 parts acetone, 8.7 parts methanol, and 78.9 parts 3A ethyl alcohol. All solutions were prepared with continuous agitation. Seven-tenths part of the first solution was then slowly added to 10.9 parts of the second solution with continued agitation. The mixture of the first solution and the second solution was then slowly added to 73.1

parts of the third solution which was heated to 50 C.

Mild and continued agitation was maintained and the mixture thus formed Was allowed to react for 25 minutes at the 50 C. temperature after which it was cooled. When the temperature of the reaction mixture reached 35 C., 14.5 parts of n-butanol was added and the re The resulting composition had a green color, a solids content of 12%, and a viscosity of 25 seconds (Ford No. 4 cup).

Compositions in accordance with this invention which were prepared as set forth in the foregoing example were applied to metal panels. The panels tested were aluminum (3S) and cold rolled steel. The panels were first thoroughly cleaned insolvent, then one-half of each panel was dipped in the wash primer of this invention using a dip coater adjusted to provide a 0.3-0.5 mil dry film. One set of primers on each metal was air-dried before topcoating; the other set was baked 30 minutes at C. After the designated drying schedule, the entire panel was sprayed with three coats of nitrocellulose lacquer.

Impact resistance tests and scratch adhesion tests carried out on the various sets of panels showed generally an improvement in impact resistance and in scratch adhesion of the film applied over the wash primer of this invention in comparison with the film applied over the bare metal. Thus, in addition to the known corrosion resistance imparted by wash primers, wherein phosphoric acid and chromic acid have been disclosed as reacting with a metal surface to deposit a tightly adhering and insoluble layer of metal salt, there is provided an excellent filmforming constituent which is nitrocellulose bound in some manner to form a tightly adhering complex of the whole composition to metals. The complex chemical reactions which occur during the preparation of the compositions of this invention are not known. Additionally, the exact mechanism by which the compositions of this invention are bound to metals is not known. However, it has been observed physically that the nitrocellulose film or complex thereof is smooth, uniform, continuous and adhesive which is an important requisite for a corrosion inhibiting coating which also serves as a ground or base coat for painting.

It is evident that there are several factors which will influence conditions for the most satisfactory operation of this invention, the actual limits of which can be determined only by a detailed study of each set of materials to be utilized in preparing the compositions and the types of metals to which the compositions are to be applied. For example, the following factors will affect the suitability of the compositions of this invention for certain specific application: composition of solvent blend, water content, ratio of chromic acid to phosphoric acid and acid concentration, optimum reaction time and temperature, type and grade of nitrocellulose and type of metal to which the composition is to be applied.

Generally, however, it will be found that compositions prepared in proportions and using ingredients approximating those set forth in the example will usually give satisfactory results for general application to certain metals.

In this respect, although a solvent mixture of acetone methanol, and 3A alcohol is preferred as set forth in the example, other solvents or mixtures thereof may be used. Similarly, although SS nitrocellulose 5-6 sec. is preferred as set forth in the example, other protective-coating type nitrocellulose having a viscosity from about 20 to about 10,000 cps. may be used within practical .limits for obtaining the reaction product and coating composition of this invention. Water is necessary as the solvent for chromium trioxide in order to avoid spontaneous combustion which may'occur when chromium trioxide is mixed with alcohol. Acetone is preferred for preparing the phosphoric acid solution, although other similar solvents may be used. The amount of chromium trioxide in the reaction mixture should be less than the amount of phosphoric acid present and, based on 1000 parts by weight of the total reaction solution, the chromium trioxide, phosphoric acid and nitrocellulose may be varied within the following ranges: 0.5 to 5; 1 to 20; and to 200 parts, respectively. The concentration of the reaction product of this invention based on the total weight of the coating composition may be varied from about 3 to about by weight. The primer of this invention is usually used without the addition of pigments, resins or diluents, but they can be added for particular uses. Generally, however, the coating composition will be used on metals without addition of these other materials since it is quite common practice to apply a protective coating over the primed metal base. Such protective coatings may be applied in the usual manner such as by dipping, spraying, and brushing. However, the primer coating composition of this invention may be applied using the above conventional methods of application and suitably dried at room or elevated temperature. Additionally, the compositions may be applied to a metal base by dipping or immersion with or without residence time and then suitably dried.

Certain uses for the coating compositions of this invention reflect its advantages in giving a smooth, uniform, continuous and adhesive film and include more particularly: automobile finishes in place of or supplementing Bonderizing (phosphoric acid treatment); pretreatment conditioner or pigmented primer for baking systems such as refrigerators or steel cabinets; pretreatment on castings or steel moldings where corrosion would be prevented during aging and undercoat provided; coating material for sheet steel to prevent corrosion in transit and storage and undercoat provided for any desired subsequent finishing; and coating material for aluminum foil to improve the adhesion of printing inks, and other aluminum objects such as venetian blinds, window frames, door frames, house siding, trailers, boats and airplanes where improved adhesion and corrosion resistance are desired.

What I claim and desire to protect by Letters Patent is:

1. A coating composition comprising the reaction prodnot of a solution of 0.5 to 5 parts by weight of chromium trioxide, 1 to 20 parts by weight of phosphoric acid, and 10 to 200 parts by weight of nitrocellulose dissolved in sufficient organic solvent consisting principally of a mix ture of lower aliphatic alcohols and acetone to make 1000 parts by weight of total reaction mixture, the amount of said chromium trioxide being less than the amount of said phosphoric acid, said composition being characterized by improved adhesion to metals and the ability to inhibit the corrosion of metals.

2. In combination, a metal base and as a coating therefor the composition as defined by claim 1 applied thereto and dried thereon.

References Cited in the file of this patent UNITED STATES PATENTS 2,170,658 Hucks Aug. 22, 1939 2,371,866 Barrett Mar. 20, 1945 2,488,651 Whiting Nov. 22, 1949 2,562,118 Osdal July 24, 1951 

1. A COATING COMPOSITION COMPRISING THE REACTION PRODUCT OF A SOLUTION OF 0.5 TO 5 PARTS BY WEIGHT OF CHROMIUM TRIXOIDE, 1 TO 20 PARTS BY WEIGHT OF PHOSPHORIC ACID,AND 10 TO 200 PARTS BY WEIGHT OF NITROCELLULOSE DISSOLVED IN SUFFICIENT ORGANIC SOLVENT CONSISTING PRINCIPALLY OF A MIXTURE OF LOWER ALIPHATIC ALCOHOLS AND ACETONE TO MAKE 1000 PARTS BY WEIGHT OF TOTAL REACTION MIXTURE, THE AMOUNT OF SAID CHROMIUM TRIOXIDE BEING LESS THAN THE AMOUNT OF SAID PHOSPHORIC ACID, SAID COMPOSITION BEING CHARACTERIZED BY IMPROVED ADHESION OF METALS AND THE ABILITY TO INHIBIT THE CORROSION OF METALS. 